The cell cycle is regulated by a molecular control system that triggers and coordinates key events. It is driven by a built-in clock involving cyclical fluctuations of cell cycle control molecules like cyclins and protein kinases. The cell cycle proceeds through checkpoints at the G1, S, G2, and M phases and is regulated by internal and external signals. External factors like growth factors and cell density can influence the cell cycle by triggering signaling pathways that allow or inhibit progression through the checkpoints. Dysregulation of the cell cycle can lead to cancer if cells divide excessively and evade programmed cell death.

1. Concept 12.3: Regulation of
Cell Cycle by a Molecular
Control System
Will S.
Esther P.
Michelle K.
Alex G.
Nick B.

2. Evidence Of Cytoplasmic Streaming
Timing and the rate of cell division in plants and
animals is crucial to normal growth, development,
and maintenance
Some of the most specialized cells do not divide in a
mature human—these cell cycle differences are from
regulation at the molecular level

3. What drives the cell cycle?
One hypothesis—each event triggers the next;
replication of chromosomes in S phase causes cell
growth during G2 phase triggering onset of mitosis—
not correct
Cell cycle is driven by specific molecular signals
present in cytoplasm
Example: mammalian cells grown in culture

4. Example
S and G1 –G1 went to
the S phase and DNA
was synthesized
M and G1 –G1 began
mitosis- spindle
formed and
chromatin condensed
even though
chromosomes were
not duplicated

5. The Cell Cycle Control System
Operating set of molecules that triggers and
coordinates key events in the cell cycle.
Driven by a built-on clock
Regulated at checkpoints by internal and external
controls.
Checkpoint = critical point where stop and go-ahead
signals regulate the cycle

6. Continued
Signals come from surveillance inside the cell
Three major checkpoints are G1, G2, and M phases
G1 is the most important and the cell will most likely
complete the cycle if given go-ahead by G1
Not given go-ahead then leaves into the G0

7. G0 State
G1 S
M G2

8. The Cell Cycle Clock
Rhythmic fluctuations in the abundance and activity
of cell cycle control molecules pace the sequential
events of the cell cycle.
Protein kinases are enzymes that activate or
inactivate other proteins by phosphorylating them.
To be active, such a kinase must be attached to a
cyclin, a protein that gets its name form its cyclically
fluctuating concentration in the cell.

11. Shuts down by cyclin
degradation

12. Stop and Go Signals
Ensure mitosis properly
carried out
“Stops” if parts not there

13. Cells examined for normal
growth, space limitations,
growth factor withdrawal,
DNA damage, etc.

14. Look for DNA
damage, mismatched
bases that could have
happened during S
phase (DNA synthesis)

15. • metaphase ensures all
chromosomes
connected to mitotic
spindle at kinetochore
• then can go to
anaphase

16. External Factors
Influences
division
Binding PDGF to
Physical or receptor
chemical tyrosine kinases
triggers
Growth Factors transduction
Ex. PDGF pathway, allows
passage of G1
Density-
Dependent
Inhibition
Anchorage
Dependence

18. Cancer
When cells divide excessively and invade other tissues
and /or take up resources.
Apoptosis: process of programmed cell death
Normal mammalian cells divide only about 20-50 times.
Transformation: when normal cells transforms into a
cancer cell.
Benign tumor: when cancer cells stay in original site.
Malignant tumor (metastasis): when the cancer spreads
and proliferates in various parts of the body.

21. Treatment
Tumors are treated with high powered.
Chemotherapy: toxic drugs that impede the cell cycle
and stop cells from dividing.
There is a cell line in culture that has been
reproducing since 1951

24. Helpful Videos
http://youtu.be/1EB8q9aR8Hk
http://www.youtube.com/watch?v=QGx50C1w8YY

25. ??????
Questions?